Refactored the way that osg::Image/ImageSequence manages the update callback that needs to be attached to Textures to make it possible to use the Image::update() mechansim in other subclasses from osg::Image.
To enable the automatic attachment of the required update callback to call osg::Image::update(..) subclasses from osg::Image will
need to implement the osg::Image::requestUpdateCall() and return true, and implement the osg::Image::update(NodeVisitor?*) method to recieve the update call during the update traversal.